Return meridian in correct case.

According the format of meridians http://apidock.com/ruby/Time/strftime are 

```
%P - Meridian indicator, lowercase (``am'' or ``pm'')
%p - Meridian indicator, uppercase (``AM'' or ``PM'')
```

Currently it was always `am` or `pm`.

```
I18n.l Time.now, format: '%p' # => Returns 'am' or 'pm'
```